The calculation method of normal effective emissivity of double cavities blackbody at isothermic and unisothermic states is obtained by means of theoretical derivation. The effective emissivity of "Double Cavities High Precision Extended Source Blackbody" used in the infrared radiation calibration of the FY-1 Meteorological Satellite is calculated. The effect of temperature difference on the effective emissivity at unisothermic state is given quantitatively.